Nodes: Lighten/Darken "Crossfader"
Try this:



This mixes between the input and black (0) for the 0-.5 range, then mixes between the input and white (1) for the .5-1 range.

The "Lerp" nodegroup is as follows:



This multiplies one value by an inversion of the "Fac", and the other by the fac itself. Then adds the two together.

0 as the "Fac" returns the top input, while 1 as the "Fac" gives the bottom input. Values in between give a mix.

Note that this should work for extrapolating as well; "Fac" values outside of 0..1 will give a result "past" the input values. If you don't want that, clamp the Fac input to 0..1.